Natural reducing materials—including iron oxides, sulfides, and clays—have been extensively studied because of their role in iron and carbon cycling, microbial respiration, and sequestration of contaminants. In laboratory studies, the reactivity of these materials is often modeled using high purity powders that are commercially available as reference materials or freshly prepared using standard protocols. Less frequently, reducing materials such as magnetite have been harvested from environmental samples, and the range of tools that have been applied to characterization of these natural samples is limited. We have developed an approach to studying the redox properties of reducing materials that is applicable for reference and natural samples, and provides control over grain packing and pore space. The approach is based on the powder disk electrode (PDE) design we used in previous studies of zerovalent iron, but with the addition of packing the PDE with agarose-stabilized electrolyte. Agarose stabilized PDEs have been prepared with reference samples of iron oxides and magnetic material harvested from Columbia River sediments. The electrodes were characterized with chronopotentiometry, linear sweep voltammetry (LSV), and electrical impedance spectroscopy (EIS). A new equivalent circuit model was developed that allows translation of complex impedance data into parameters that correspond to reactivity on the material surface. Results of this analysis applied to the magnetic materials sourced from the Columbia River show less reactivity (e.g., smaller corrosion currents) than reference-grade magnetite other iron oxides. The diminished reactivity is attributable to formation of a passivating layer on materials while in the environment. One consequence of the passivation of these materials is likely to be lower reactivity with contaminants, which means that laboratory studies of contaminant degradation done with high-purity standards may result in over estimation of rates of in situ chemical reduction.

Characterizing the reactivity of naturally occurring reducing materials with agarose-bound powder disk electrodes

249th National Meeting (2015)

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